Pour into prepared bowl or mold and place into large shallow pan and place on middle rack in oven. Fill the shallow pan half-full with boiling water and slowly steam pudding in oven at 325°F (160°C) for 4 hours, replacing water as needed.

For Custard Sauce: In saucepan, scald milk and allow to cool.

Mix together remaining ingredients, except for butter. Add to cooled milk. Cook over low heat until thickened. Remove from heat and stir in butter, mixing well.